1. Field of the Invention
This invention relates to a charging device in an image forming apparatus and, more particularly to a charging device which charges a charged body by moving a charging member close to the charged body in non-contact state.
2. Description of the Prior Art
Image forming apparatuses including optical printers such as copiers, laser printers, electrostatic image recording apparatuses and the like, use a photosensitive body, a dielectric body or the like as an image carrier to be charged in image formation. That is, the image carrier such as the photosensitive body or dielectric body is a charged body. As a charging device to charge the charged body, a corona-discharge type charging device using a corona wire is known.
In recent years, a pin-discharge type charging device using a metal plate having a number of sharp edge portions on the charged body side, i.e., so-called saw-tooth electrode plate has been proposed. This charging device performs corona discharge from the end of the sharp edge portions. Japanese Patent Application Laid-Open No. 8-106198 discloses the corona charging device with a saw-tooth electrode.
Further, in recent years, contact-type charging devices such as a brush charging device, a roller charging device or a blade charging device, which charge a charged body by bringing a charging member into contact with the charged body, have been proposed and put into practical use. These contact-type charging devices perform charging by bringing the charging members such as brushes, rollers or blades and a blade, connected to a power source output, into contact with the surface of an image carrier as the charged body.
However, the above-described respective charging devices have the following problems. The corona charging device using the corona wire needs a high-voltage power source having the absolute output value of 4-7 kV and space for providing a shield around the wire. This causes problems when treating the device or performing wiring and setting the size of the device. Further, as corona discharge by a high voltage causes a large amount of corona products such as ozone, it is necessary to take measures to cope with the corona products. Further, the discharge by using a wire causes discharge unevenness in a wire lengthwise direction, thus easily causes charging unevenness. The charging unevenness is also caused by contamination of the wire. The charging unevenness can be improved by a scorotron charging device having a grid electrode between a discharge wire and a discharged body, however, in the scorotron charging device, the wire voltage must be higher, therefore, the occurrence of ozone and the like increases.
As a compact charging device to replace the corona charging device using the corona wire, a pin-discharge type corona charging device using a needle electrode or saw-tooth electrode has been proposed. However, the pin-discharge type charging device still needs a high-voltage power source similar to that of the corona charging device. Further, as discharging portions are arranged at fixed intervals, charging unevenness easily occurs. This charging device cannot be applied to high-density and high-resolution image formation, which must be further improved. When foreign materials are attached to the end portion of the sharp discharging portions or any mechanical defect occurs, charging uniformity is a more serious problem. Similar to the above-described scorotron charging device, a charging device with a grid electrode has been proposed to improve the charging unevenness in the pin-discharge type discharging device, however, it is not effective to obtain charging uniformity.
Japanese Patent Application Laid-Open No. 8-106198 proposes a charging device using a saw-tooth electrode and a grid electrode, where the end of the saw-tooth electrode plate is opposite to the shield so as to obtain a small and uniform charging characteristic. However, in this charging device, a high voltage of -5 kV must be applied to the saw-tooth electrode, which causes a large amount of ozone and the like. Further, even though the size of this device is smaller than that of the wire-discharge type device, the downsizing has not been made on a large scale. Accordingly, an image forming apparatus using this device needs large space, in consideration of measures against corona products.
Japanese Patent Application Laid-Open No. 61-99172 proposes a construction of a scorotron charging device in which the grid electrode is divided in an axial direction of a photosensitive body into several portions to respectively receive an applied potential. The grid portion having a first electrode, an insulating layer and a second electrode controls selective charging with respect to the photosensitive body. However, for the purpose of charging uniformity, the respective layers having an opening pattern must be precisely deposited. Further, if the grid portion has a three layer structure, higher deposition precision is required. This increases the price of the product. Further, wiring must be made on the respective electrodes, which also increases the price. As the size and arrangement of the opening patterns influence the charging uniformity, the opening size and the opening pattern arrangement cannot be optimized without difficulty, and practical patterns cannot be designed without difficulty. Further, in a case where the grid has the second electrode to perform charging/not charging control, as the opening pitch and arrangement of the opening patterns influence the image forming resolution, this structure cannot be applied to high-resolution image formation without difficulty. Further, the device basically usels a corona wire, it has problems regarding its high-voltage power source, device size and corona products, similar to those of the above-described corona charging device.
In recent years, the contact-type charging devices widely used in small laser printers and the like also have problems. In the use of brush charging, it is difficult to realize a stable charging uniformity through manufacture of the brush, attachment of the brush, determination of power application condition and the like. This may cause charging failure, and cannot obtain high reliability. In the use of roller charging, the problems in the charging uniformity and reliability are reduced since manufacture and attachment of the roller are easier in comparison with the case of the brush, however, there are still problems in deterioration of charging performance when the roller surface has a flaw or foreign material is attached to the roller surface and in image formation at a high speed. In the use of blade charging, as the same linear portion is always in contact with a charged body, maintenance of the characteristic of the contact portion is very significant.
This poses problems in reliability, cost, and image formation at a high speed.
Further, it can be generally considered in all the contact-type charging devices that, if there is a defect such as a pin hole on the charged body side, spark discharge occurs and charging failure occurs in the lengthwise direction including the pin hole. Furthermore, when a multi-color image is formed on the image carrier such as a photosensitive body, the contact-type charging disturbs an image formed on the image carrier, and the image cannot be used.